Journal of Tropical Ecology (2010) 26:335–341. Copyright © Cambridge University Press 2010 doi:10.1017/S0266467409990654 Honeydew foraging by birds in tropical montane forests and pastures of Mexico Heather A. Gamper 1 and Suzanne Koptur Department of Biological Sciences, Florida International University, Miami, FL 33199, USA (Accepted 4 December 2009) Abstract: A honeydew-producing scale insect, Stigmacoccus garmilleri (Margarodidae), is associated with oak trees (Quercus spp.) in highland forests of Mexico. Although feeding by ants on scale-insect honeydew is more frequently documented in the literature, the honeydew produced by feeding instars of S. garmilleri is sufficient to provide nourishment for birds. This study elucidates bird use of honeydew in the tropical montane forests near Chiconquiaco, Veracruz, Mexico, and uncovers patterns in honeydew foraging. Over a 2-mo period, 40 trees harbouring scale insects, located in both forest and pasture areas, were intensely studied (160 h of bird-foraging observations along with quantitative measurements of honeydew production). Fifteen resident bird species and 18 migrant species were observed visiting observation trees. Approximately 72% of the resident bird species and 83% of the migrant bird species observed were recorded to forage on scale-insect honeydew. Audubon’s warbler (Dendroica coronata auduboni) was the most active consumer and defender of the resource. Of 118 aggressive chases observed, only 9.65% occurred in forest observation trees, and 90.3% in pasture trees. Audubon’s warbler demonstrated preferential defence and territorial patrolling of scale-insect honeydew in scattered pasture trees. Key Words: avian, land use change, Margarodidae, plant–animal interactions, resource defence, scale insects, scattered trees, Stigmacoccus INTRODUCTION Honeydew is a sugary excretion of phloem-feeding insects. It contains large amounts of carbohydrates and trace amounts of amino acids and can provide an important food source. Honeydew-producing insects tend to excrete copious honeydew, live in groups, and are typically sedentary or semi-sedentary (Williams & Williams 1980). Scale insects (Coccoidea) are well known for their honeydew secretions and relationships with ants that feed on them (reviewed by Bach 1991, Buckley 1987, Way 1963). To a lesser extent, the use of scale insect honeydew by birds has also been documented (Beggs 2001, Gaze & Clout 1983, Greenberg et al. 1993, Jir ´ on & Salas 1975, Koster & Stoewesand 1973, Latta et al. 2001, Murphy & Kelly 2003, Paton 1980, Reichholf & Reichholf 1973, Woinarski 1984). However, the dynamics of the 1 Current address: Department of Geography, Florida State University, Tallahassee, FL 32306-2190, USA. Email: hgamper@fsu.edu interaction between scale insects and birds remains poorly understood. Honeydew may provide an important resource for nectarivorous and frugivorous birds, especially during times when flowers and fruit are not abundant. Such a resource may be worth defending, just as large displays of nectar-producing flowers (Feinsinger 1976, Mac Nally & Timewell 2005) and fruit (Male & Roberts 2002, Pratt 1984) have been shown to promote territorial defence in birds. Migratory birds do not commonly engage in interspecific aggression in wintering habitat (Greenberg et al. 1996), but Greenberg et al. (1993) found that where such aggression occurs, it is most common when birds are using resource-rich patches such as fruit or nectar. Several studies have documented bird defence of honeydew (Greenberg et al. 1993, Latta & Faaborg 2002, Latta et al. 2001, Paton 1980, Woinarski 1984). Along with the type and richness of the food involved, habitat structure may play an important role in the defensibility of a resource. Single trees in open pasture may be more defensible than a structurally diverse forest with a greater abundance of birds (Orians & Willson 1964).